The present invention relates generally to broadband and wireless communications and more particularly to precoding selections for retransmissions in uplink multiple-input multiple-output MIMO Hybrid automatic repeat request HARQ.
Uplink (UL) multiple-input multiple-output (MIMO) with linear precoding has been considered as one important feature for the enhancement of UL transmission in new LTE-Advanced (LTE-A) standard for the fourth generation (4G) cellular systems. The feasibility studies for UL precoding in single user MIMO have shown that an approximately 3 dB gain can be achieved by the precoding over the no precoding transmission for a codebook size of 16. After extensive discussions, the precoding codebook for the UL MIMO has been finalized in for up-to-four transmit antennas.
On the other hand, synchronous non-adaptive hybrid ARQ (HARQ) is still the basic principle for LTE-A UL transmissions, which is the same as that in the LTE systems. In the synchronous non-adaptive HARQ, the retransmission is scheduled in the fixed time instance and the fixed resource block with the same modulation and coding scheme (MCS) as that of the original transmission. The adaptive retransmission is only used as a complement to non-adaptive HARQ to avoid fragmenting the uplink frequency resource or to avoid collisions with random-access resources. The physical hybrid ARQ indicator channel (PHICH) carries the acknowledgement (ACK or NACK) and is transmitted from the base station to the user equipment (UE). Without additional information, the synchronous non-adaptive UL retransmission is operated by default. When adaptive retransmission is scheduled, the resource block and MCS information are delivered to the users through the physical downlink control channel (PDCCH) for the uplink retransmissions.
Since, in LTE systems, the uplink MIMO is not supported, only one codeword is transmitted for each time interval. Thus, the problem regarding the multi-codeword MIMO in hybrid ARQ as that for the DL the hybrid ARQ does not exist. However, this is not the case in LTE-A systems when the uplink MIMO is introduced. Therefore, non-adaptive UL retransmission triggered by PHICH in LTE should be extended to multi-codeword transmission in LTE-A UL. For multi-codeword transmission in uplink MIMO, when the number of codewords in the retransmissions is different from previous retransmissions, how to assign the precoding for the retransmissions is a problem. Here we assume that at most two codewords are allowed for transmission across multiple layers in UL MIMO. Based on the latest discussions in 3GPP meetings, when the number of codewords in the retransmissions is same as previous retransmissions, the rank and precoding vector stay the same, and the retransmission does not carry any automatic power adjustment command.
Several schemes have been proposed for the precoder selections for non-adaptive HARQ in UL MIMO: 1) User terminal selects whatever the precoder wants; the basestation can still decode since the demodulation reference signal (DMRS) is precoded; 2) Use of subset of columns of a precoding matrix with the one corresponding to larger Modulation and coding scheme (MCS); 3) Use of predefined precoding matrix (fixed or cycle) among a set of precoding matrix; 4) a single antenna transmission mode; 5) Using the same precoding matrix as that in the original transmission and occupying all layers; and 6) Precoding column compression with a merger of the columns.
However, none of the aforementioned techniques address the problem of precoding selections at the user terminal for retransmitting one or multiple codewords in non-adaptive hybrid ARQ for uplink (UL) MIMO when the transmission rank or number of codewords in the retransmissions is smaller than the corresponding value in the original transmission. Particularly, the problem framework considers that only imperfect channel state information (CSI), e.g., quantized channel information, is available at the user terminal. Accordingly, there is a need for precoding selection for retransmission in uplink hybrid ARQ that solves this problem.